ST. LOUIS -- After weeks of being patient with the development of Alex Reyes and Luke Weaver, the Cardinals are finally ready to debut their top two prospects.

Reyes arrived first, donning a big league uniform on Tuesday after the Cardinals purchased his contract from Triple-A Memphis. He appeared in the ninth inning of Tuesday's 7-4 loss to the Reds and pitched a perfect frame, striking out his first batter. But it will be Weaver who draws the first start, as the Cardinals tapped him to replace Michael Wacha -- who was placed on the disabled list -- in the rotation. His debut will come at Wrigley Field on Saturday.

"It went from zero to 100 when I got the call last night from my [Triple-A] manager," said Reyes, who learned the news while out for dinner. "It was a crazy experience, a dream come true, and I'm happy to be here. I don't know what it'll feel like once I step out there."

Reyes, 21, has been pitching for Memphis since serving a suspension for a positive test for a drug of abuse at the start of the season. He has a 4.96 ERA in 14 starts and most recently pitched on Saturday, when he went six innings and allowed one run on six hits. He's struck out 93, but also walked 32, over 65 1/3 innings.

Reyes' first Major League K

CIN@STL: Reyes records his first career strikeout

In his Major League debut, Alex Reyes whiffs Adam Duvall swinging for his first career strikeout

That walk rate -- and efficiency issues -- have been cited as reasons why Reyes was not in position for a promotion earlier in the season. It's also why the Cardinals felt more comfortable handing the starting assignment over to Weaver, who has averaged better than six innings a start and has a walk rate of 1.3 per nine innings this season.

"I feel like when you look at the ability to throw strikes, get deeper into games, I think that's important," general manager John Mozeliak said. "Candidly, [Weaver] only had one start in Triple-A, but I think he'll still give us a good opportunity to win that game."

Top Prospects: Weaver, STL

Top Prospects: Luke Weaver, RHP, Cardinals

2016 MLB.com Top Prospects: Luke Weaver's velocity bounced back in 2015, with his fastball consistently sitting around 94 mph

Weaver, ranked by MLBPipeline.com as the No. 93 overall prospect, went six strong innings in that Triple-A debut on Monday. In 13 Minor League starts this season, the 22-year-old is 7-3 with a 1.30 ERA.

"He's had great reports for a while, but it just looks like this last year has been a different level," manager Mike Matheny said. "Wherever he's gone, whatever kind of challenge he's been given, he's been able to answer it."

Mozeliak didn't rule out the possibility of Weaver and Reyes piggybacking one another at some point, though that will depend upon how Reyes is deployed out of the bullpen. Some upcoming off-days will also give the Cardinals an opportunity to readjust assignments, if needed.

Reyes has the ability to crack triple digits on the radar gun and complements that fastball with a changeup and curveball. With Trevor Rosenthal on the DL, the Cardinals don't have another power arm of that type in the 'pen.

"He's as advertised," Mozeliak said. "He's got legitimate horsepower. I think the big thing for him is, 'Can he command the strike zone at the big league level?'"

Reyes strikes out four

WLD@USA: Reyes strikes out four in 1 2/3 frames

World pitcher Alex Reyes strikes out four batters in 1 2/3 innings of work in the Futures Game

Utilizing Reyes as a reliever until further notice has not diminished the organization's expectations for him long term. The Cardinals still believe he's a future front-line starter, but they've had success in transitioning starters to the Majors via the bullpen before. Current starters Adam Wainwright and Carlos Martinez both took that path.

"I feel like if you make pitches down in the zone you should be able to make pitches anywhere," Reyes said. "I'm confident in my stuff and can hopefully get quick outs."

In order to clear a 40-man roster spot for Reyes, the Cardinals transferred Brayan Pena to the 60-day DL. Another roster move will have to be made ahead of Saturday's game to add Weaver.

Jenifer Langosch has covered the Cardinals for MLB.com since 2012, and previously covered the Pirates from 2007-11. Read her blog, follow her on Twitter, like her Facebook page and listen to her podcast. This story was not subject to the approval of Major League Baseball or its clubs.